Transformer



l.. 1 JoNEs 1,901,368

TRANSFORMER Filed Oct. 25, 1 929 3 Sheets-Sheet l March 14, 1933.

INVENTOR eser L. Jones L. L. J ON ES I TRANSFORMER March 14, 1933.

Filed Oct. 25, 1929 3 Sheets-Sheet 2 )OZMDUMMM NOLLVDLQVIdWV ATTORNEYS 14, L L JONES TRANSFORMER Filed Oct. 25, 1929 3 Sheets-Sheet 3 ATTORNEYS Patented Mar. 14, 1933 y 1.. JONES, or OEADEEL, NEW JERSEY 'rEaNsFoRMEE Application led October 25, 1929. Serial No. 402,379;

This invention relates to radio circuits and more particularly to inductancel coils Aand transformers used in suchl circuits. l

Inductance `coils are frequently used in radio circuits and it is often necessary to decouple such coils from one another and from other parts of the neighboring circuits in order to prevent undesired coupling or reaction. This result may be obtained by complete metallic shielding of .the individual coils, but in many cases such practice is not desirable because of the capacity shunting effect of the metallic structure which may reduce the eiiciency of the circuits and also be cause of the cost of the shielding. Another and often far more satisfactory solution is to wind the coils astatically, which is most simply done by dividing the coil into two equal portions which are wound in opposite directions so as to decouple the coil from other neighboring circuits, and which at the same time are sufficiently separated'to avoid complete field opposition, so that the total inductance of the two coils connected in series is equal to the desired inductance value. Such a transformer is disclosed in my Patent No. 1,732,937, issued October 22, 1929.V

When this is done undesired resonance peaks may appear in the operation of the circuits involved when they arel used over a desired or working range of frequency. I have found that this is caused by reason of the individual sections of the divided coil having a natural resonance frequency which falls inside of the working frequency range. The primary object of my invention is to overcome this dificulty, and to provide an astatic coil which will not result in undesired reso# nance peakswithin a predetermined range of frequency, which I do, broadly, by subdivid ing the coil into a plurality of series connected sections such that each section vhas a natural frequency outside' of the yfrequency range. In theV usual case the complete coil is of such dimension that the natural fre# quency thereof falls below the frequency range, and it is desired to avoid all resonance peaks within therange. In dividing the coil into two sections in order to obtain astaticism each of the sections will have a natural frequency higher than that of the complete coil, and this: frequency may come within* the working frequency range.y In accordance with the presentinvention the coil will then e be divided into more than two sections, and may most simply be divided into an even number of equal sections, say lfor 6 sections, such that each section will have a natural frequency higher than thel working frequency range. Alternate sections are wound in opposite directions, thereby retaining the astaticism of the original divided coil, and in fact improving the astaticism thereof.

The foregoing difficulty frequently arises in connection with the coupling transforiners in cascaded electron emission tube amplis fiers, and particularly radio frequency amplifiers of the untuned type. If it is desired to avoid resonance peaks in the operationof such an amplifier the transformers, thereof may be selected to have natural frequencies lying outside ofthe range of frequencies to be amplified. .This frequency may be selected above or below the working frequency range, according to which way it is preferred to have the amplifier gainjcurve slope. i In most cases the natural frequency of the transformers is lower than that of the working frequency range, and ifr it is attempted to prevent interstag'e transformer coupling by making the 80 transformers astatic in accordance with the disclosure of my said' Patent No. 1,732,937 the difficulty of resonance peaks within thel working frequency range may arise. In accordance with my presentinvention, this difficulty may be overcome'in a manner similarl to that already mentioned, that is, by subdividing the transformer into a-greater number of'alternately oppositely wound sections than two, each of the sections having a natural*V frequency higher than the Working range kof frequencies.

In `my copending application Serial No. 397,633, filed October 5, 1929, since issued on October 28, 1930, as PatentvNo. 1,779,881, I yF have disclosed how a cascade amplifier having fixed circuit constants may bel arranged to be automatically tuned over a broad range` pling the successive tubes with transformers of successively increasing inductance. yIn such an amplifier 1t isv customary to make the transformers with substantially unity cou-V pling, preferably by Winding the transformers bifilarly as described in my said Patent No. 1,733,937, in order to obtain a smoothly varying vimpedance characteristic for the transformers and therebyto obtain smoothlyV Varying amplification and reaction characteristics for the amplifier. If the transformers are subdivided into oppositely wound halves for the sake of astaticism those'of'greater in'-` ductance rhave halves which arenaturally resonant to frequencies within. the working frequency range, thereby causing undesired sharp variations in the impedance characterf istic of the amplifier, and my invention is particularly applicable to such transformers, it being merely necessary to providey an. increased number of alternatelyV oppositely wound sections, each of the sections being Wound with both primary and secondary conductors whichmay be preliminarily twisted together with a relatively long pitch so as to obtain the desired bifilar coupling;

To the accomplishment of the foregoing and suchother objects as will hereinafter appear, my invention consists in the coil and transformer elements, the amplifier circuit :sor

embodying the same, and their relation one to the other as hereinafter are more particularly describedv in the specification and sought to be defined in the claims. The specification isaccompanied bykdrawings in Which: v

Fig. 1 is a wiring diagram for a receiver including 'an automatically tuned amplifier y embodying transformers constructed in accordancel with the present invention Fig. 2 is a graph showing the effect of my invention as applied to transformer 24 in Fig. 1; f n' Fig. 3 is a graph showing the effect of my invention as applied to transformer 36 in Fig. 1; l

Fig. 4 is a dimensioned elevation of an insulation spool suitable for coils 16 and 38 in* Fig. 1; v d

Fig. 5 is a dimensioned elevation of an insulation spool suitable for transformers 21, 28 and 32 in Fig. 1; i i

f suita le for transformer 24 Fi' 6 is adimensioned elevation of a spool in Fig. 1;

Fig. 7 is a dimensioned elevation of an insulation spool suitable for transformer 36 in 1g. l 1 Fig., 8 is a graph explanatory of advantages of -my invention;

F ig. 9 is a schematic diagram of a receiver v including an amp-lifier to which my invention may advantageously be applied; and Y Fig. 10 is a wiring diagram of such an amplifier.V y

ReferringV now to F ig. 1 ofthe drawings, thereis a selector having variable circuit constants for tuning, followed by an amplifier having fixed circuit constants, the arrangementbeing in general similar to that set forth in lmy copending application Serial N o. 397,633,1now Patent No. 1,779,881, previously referred to. n

Energy collected on any suitable antenna circuit V2 is fed to a first resonant'circuit-f,

which is loosely coupled to a second resonant circuit 6, which in turnV is loosely coupled to athird resonant circuitk 8 bymeans of a small auxiliary coil 10, and the yresonant circuit 8 is loosely coupled to a fourth and final resonant circuit 12. The various tuning circuits are loosely coupled in order to avoid double resonance peaks, and the circuits are suitably positioned or shielded so that non-adjacent tuning circuits are effectively decoupled.

.The received energy of selected frequency is supplied to the input circuit of a tube T1, the output circuit of which includes a coil 16, which is inductive Within the `Working frequency range and therefore causes a regenerative reaction. Resistance 18 is arranged in series with the inductance 16 in order to cause VLa feed forward ofV energy through the tube T1 whichl tendsto neutralize the regenerative reaction of the inductance 16, and

a condenser 20 is preferably connected in parallel with the resistance 18, all in accord- T2 through a Vbifilar transformer 21, the pr'- A mary of which is connected in parallel With the complex output circuit previously described, and the secondary of which isconnected across the control electrode to cathode circuit of the tube.

tubes T3, Tl and T5 nected in cascade through bifilar transformers 24, 28, and 32. The amplified output-from tube T5 is supplied to a detector tube 40 Succeeding amplifierV are Vrespectively conthrough a bifilar transformer 36, in series with the secondary of which there is connected a free grid inductance 38. The radio amplifier or a reproducer through connectionsto'the terminals marked Output.

Considering the second stage including the tube T2 morezin detail, the transformer 24 is selected to be naturally resonant when in circuit to a frequency lower than the lowest woll ring frequency in order to obtain a rising apparent input capacitance with decreasing frequency for the tube T2. The output circuit of the tube Tl is given a substantially higher impedance than would be correct for a Vreactionless stage, this being for they purpose of substantial-ly boosting the amplification gain therein, as is described more particularly in my copending application Serial No. 397,632, aforesaid. As is there set forth, I have found that botli the inductive and resistance components of the output circuit of the tube Tl may be eachincieased to a value approximately double that for producing a reactionless stage, the impedance of this circuit being doubled in effect, and this producing a substantial doubling in the gain in the tube Tl.

Doubling this impedance causes residual reaction through tube T1, and this reaction maybe made regenerative over the high frequency end of the range by increasing the feed forward resistance 1 8 slightly less than proportionally to the increase of the inductance 16. The reaction will still be degenerative at 'the low frequency end of the range because the slope of the reaction curve differs from that desired. To correct for regenerative reaction capacitance may be hung onto the anode terminal of the output circuit of tube T1.V The input circuit of tube T2 is therefore not correctly automatically tuned, but rather is made capacitive, which may be done by selecting a transformer 2l with a larger inductance than would be correct for automatic tuning, because for resonance the capacitance then should be decreased, and not being decreased, is in excess. But the effective capacitance should decrease at the high frequencies and increase at the low frequencies, for more regeneration is neededy at the high frequencies and more degeneration at the low frequencies, in order to change the slope of the reaction curve. This is accomplished by the variation in input capacitance of tube T2 obtained by proper selection. of the output transformer 24 of tube T2.. Incidentally, the gain in tube T2 is augmented by the approach to automatic tuning which is meanwhile obtained inthe input circuit of tube T2. The design of the circuits of tubesTl and T2 is set forth in detail in my aforesaid copending application Serial No. 397,632.

f. As has already been mentioned, transformer 24 is `naturally resonant to a frequency lower than the working frequency range and therefore is of considerable inductance. When this transformer is divided into oppositely wound halves, as was done with transformer2l for the sake of astaticism, the natural frequency of each section or half falls within the working frequency range and causes an undesired and rather sharp inflection point in the impedance characteristic of the transformer whichcauses a corresponding inflection point in the input capacitance curve of tube T2. This in turn causes an inlection point in the reaction characteristic of tube T1 which is undesired. More lspecifically, resonance in transformer 24 causes increased impedance which causes increased apparent input capacitance in tube`T2 which causes increased feed forward through tube Tl, or degeneration and damping in selector circuit 12. c y

T his is shown in the graph of Fig. 2 of the drawings, in which curves showing micromicrofarads detuning needed in a sharply resonant circuit, such as the selector circuit l2, in order to reduce the voltage thereacrcss to one half of its value,.are plotted as a function of the wave length of the energy being supplied to the circuit l2.` The manner in which this test may be made is set forth more in detail in my copending application Serial No. 397,632, previously referred to. When transformer 24 is wound astatically in two sections a curve such as the curve A in ig. 2 results, this curve showing a sharp upward peak in the working range of frequency. This indicates that although the transformer as a whole is resonant to a frequency lower than the working frequency range, when subdivided the natural frequency of each of the halves,

while a much higher frequency than that of the entire transformer, is not sufficiently high tol come above the working frequency range, and coming within the working frequency range causes an undesired abrupt inflection point in the reaction curve. The peak being upward indicates considerable detuning to be necessary to reduce the voltage across the resonant circuit, showing that the sharpness of tuning of the resonant circuit has been greatly diminished by a degenerative or damping reaction upon the circuit, as was previously predicted.

In accordance with my present invention transformer 24 is in this case ywound in six equal sections, alternate sections being wound in opposite directions. When this is done the natural frequency of the sections is moved out of the working frequency range, and a reaction curve like the dotted curve B in Fig. 2 is obtained.

The remainder of the stages in Fig. llmay matic tuning of tube T5 over as best be described starting at the end of the amplifier and working backward. 4The de-k tector tube having a low impedance output circuit for radio frequency energy has an apparent input capacitance whichV is small and fixed. This yis most suitablevfor automatic tuning, andtransformer 36 is selected quencies', which decreases the radio frequency potential onthe anode of tube'T, and consequently causes adecrease in apparent input capacitance withy increasing frequencies, as isy desired for automatic tuning. For the available range of variation in capacitance the transformer 32 is selected to obtain autolarge a portion of the range as is available beginning at the lowfrequency end of the range.

Transformer 32 is not suitable for automatic tuning lof Athe input circuit of tube T4 over the same frequency portion because transformer 32 is kept in resonance to such frequencies. Howeveig this transformer is related to frequencies higher than the .portion to which transformer 32 is automatically tuned just as transformer 36 is related to the lower frequency portion, that is lto say, transformer 32 is naturally resonant to ay frequency lower Vthan the portionV of the range of frequencies to which the tube T4 is to be automatically tuned. Therefore, for frequencies lying inV the higher frequency portion of the Working frequency range, the outputfcircuit .of tube T4 is capacitive-and therefore decreases .in impedance with increasing frequencies and so causesV the decrease in the apparent input capacitance of tube T4 with increasing frequencies which is desired for automatic tuning. Transformer 28 is of smallv inductance, selected to resonate with the range of apparent input capacitance of tube T4 to the higher frequency portion of the working frequency range. y

Variable resistance is connected in series with the anode supply lead 54 so that byk varying the resistance thereof the effective or operating anode potential applied to the anodes of the tubes may be varied. The lead 54 is preferably connected to only the tubes T3, T4, and T5, tubes Tl and T2 being sup'- plied with a constant anode potential through lead 52, it being somewhat preferable not to vary the operating potential applied'to tubes T1 and T2 in order not to vary the apparent input capacitances of these tubes, for'varia- 'tion of the input ,capacitance of Vtube Tl alters the tuning of circuit l2 and therefore diminishes .the selectivity of the'rselector,

while variation of the input capacitance jof tube T2 varies the reactionlessness kof tube T1, and therefore the selectivity of the selector. These problems are discussed more in detail No. 397,632, previously referred to.

It will further be noted that an induotanceV 38 iis connected in series with the secondary of'transformer36 and the control electrode of the detector tube 40. This inductance is selected of such magnitude as will series resonate with the apparent tube input capacitance of the detector tube at a frequency quency energy), because otherwise the amplification tends to fall off at higher frequencies. Thev free grid inductance 38 is also preferably wound astatically, as is indicated on the wiring diagram. t is not claimed in the present application, being disclosed in greater detail and claimedina copending application of Jacob Yolles and myself, Serial No. 402,378, filed October 25, 192,9. f

Transformersk 28 vand 32 are relatively small in inductance, sufficiently sorthat when divided for astaticism the natural frequency ofthe sections falls outside ofthe working frequency range. Transformer ,36, on the other hand, is resonant to a frequency lower than the working frequencv rangeV and is ratuer largein inductance, so that wheny subdivided the natural frequency of each of the halves falls within the working frequency range. The effect of this is illustrated in F ig. 3 in which curve C shows' a rather sharp depression, indicating a regen# erative o r feed-back reaction uponv the selector circuit.` v ,i

The reason the reaction in regenerative is readily explained by tracing the effect of a resonance impedancerise 1n transformer 3G onthe preceding circuits. Referring to Fig. l, resonance in transformer 36 causes an impedance rise which causes an increase in apparent input capacitance of tube T5. Transformer 32-is naturally resonant to frequency lower than the *highV frequency newy beingvconsidered, and a rise in input capacitanceofv tube T5 causes` this frequency to be still further loweredand therefore causes a still'further decrease of in my copending application Serial this case is vthan the natural resonance frequency apparent input capacitance of tube T4 causes transformer 28 to more nearly approach resonance and therefore to sharply increase in impedance, whichk in turn causes a sharp increase in the apparent input capacitance of tube T3. Transformer 24, being naturally resonant to a low frequency, is off resonance, and is put still further off resonance by the Vincrease in apparent input capacitance of tube T3, thereby causing a reduction in the impedance of transformer' 24 which in turn causes a reduction in the apparent input capacitance of tube T2. A reduction in the capacitance hanging onto the anode terminal of the output circuit of tube T1 causes feedback or regeneration, just as an increase in the capacitance hanging onto the anode terminal of the output circuit of tube T1 causes an increase in feed-forward or degeneration. Reduced damping orregeneration in tuned circuit 12 causes it to be more sharply resonant, so that but very slight detuning is needed to reduce the potentialthereacross tohalf its value, and this is indicated by the sharp depression in the curve C in Fig. 3.

In accordance with my invention transformer 36 is divided into four sections instead of two, and alternate sections are wound in opposite directions. The natural frequency of the sections is moved out of the working frequency range, and the reaction curve upon selector circuit 12 becomes like the dotted curve D in Fig. 3, which isa smooth curve, as is desired. i

It will be appreciated, of course, that inflection points in the desired smooth curve are undesirable, whether upward or downward. However, in further discussion of the difference between the curves in Fig.k2 and Fig. 3, reflection will'show .that when the transformers are all naturally resonant to a frequency lower than the working frequency range, or even to a frequency lower of the transformer section kbeing dealt with, the resonance in the transformer sections following the even numbered tubes will cause degeneration and an upward inflection point'such as is illustrated in Fig. 2, while resonance in the transformer sections following the odd numbered tubes will cause regeneration or downward inflection points such as is illustrated in Fig. 3.

This effect may be altered when the transformers are not resonant to lower frequencies than the undesired resonance frequency. For example, if in Fig. 1 the amplifier is so designed that transformer 28 is automatically tuned to frequencies as high as that correspondin g to a 200 meter wave length, the effect of undesired resonance in a section of transformer 36 may in some cases work out kas follows. Increased impedance in transformer 36 causes increased apparent input capacitance in tube T5 which lowers the resonance frequency of transformer 32 and decreases the apparent input capacitance of tube T4. Transformer 28 being in resonance is thrown off resonance by this capacitance change and is therefore decreased in impedance, which decreases the apparent input capacitance of tube T3, therebyA increasing the natural resonance frequency of transformer 24 and increasing the impedance thereof. This increases the apparent input capacitance of tube T2, and the resulting increasing capacitance hanging onto the anode terminal of the output circuit of tube Tl'causes forward or degenerative reaction in selector circuit 12, which causes an upward inflection in the detuning curve, similar to that shown in Fig. 2 for the effect of undesired resonance in transformer 24.

The following is a suitable set of quantitative design values and dimensions for the amplifier illustrated in Fig. 1. Resistance 18 may have a value of 2800 ohms. Capacitance 20 may be 24 micro-microfarads. Inductance 16 consists of two coils arranged astatically, each coil consisting of 83 turns random wound in a pair of slots on a spool such kas is illustrated and dimensioned in Fig. 4, thel outside diameter of the winding being about g of an inch, and thetotal inductance of coil 16 being 0.22 milli-h. Transformer 21 isa bifilar transformer preferably formed by twisting primary and secondary wires together with avery long pitch and random winding the-twisted wire in opposite direc-- tions in a pair of slots in order to obtainy astaticism. The transformer consists of 210 turns in each slot of a pairl of slots on a spool arranged and dimensioned asis indicated inl Fig. 5 of the drawings, the outside diameter of the winding being about 1% of an inch, and the transformer having an inductance of 1.16 milli-h. Transformer 24 is to have an inductance of about 4.5 milli-h. If wound in two sections like. transformer 21 resonance develops. If wound in four sections the natural frequency of the sections is fairly close to the high frequencyV end of the range and I therefore prefer to wind this transformer in six sections. Each section consists of 250 turns, alternate sections being wound in opposite directions in the slots of a spool such as is illustrated in Fig. 6l of the drawings, the outside diameter of the winding being about '178 of an inch.

Transformer 28 is a bifilar transformer consisting of 150 turns wound in each of a pair kkof slots on a spool such as is illustrated in Fig. 5 of lthe drawings, the outside diameter of the winding being about 5/8 of an inch, and the transformer having an inductance of 0.64 milli-h. Transformer 32, like transformer 21, is a bifilar transformer consisting of 210 turns wound in each of a afeed pair of slots on a spool such as is illustrated in Fig. of the drawings, the winding having an outside diameter of 1% ofan inch, and the transformer having an inductance` of 1.16" milli-h. Transformer 36 1ste have an 1n-v ductance of about 3.15 milli-h. If wound in two sections resonance develops within the working frequency 'range and I there- Y. fore lwind this transformer in four sections.

. described in my Patent vEach section'consists of 250 turns and Valternate sections are oppositely wound in the slots ofaspool suchas is illustrated in Fig. 7 of the drawings, thel outside diameter of the windings being about 'V8 of an inch. Inductance'38 consists of 175 turns per slot in fa pair of slots on a spool suchl as isillustrated in Fig. t of the drawings, the windings having an outside diameter of lof an inch, and the coil having an-'inductance of 1.01 milli-h. f The foregoing dimensions assume the use of alternating current heater f type of tubes commercially denoted type 227,

havinga grid to platecapacitance of say 3 micro-microfarads, an amplification constantof say13, and a tubeimpedance of say 1300ohms.

The spools in each "case are'y made of insulating material which may befamolded insulation or which may simply befimpregnated wood. The spacing betweenthe spools is'reduced, relativeto the best spacing,`as

N o.l 1,732,937 aforesaid, in order to make the transformers more compact,falthough Lthis close spacing necessitates slightly more winding for apredetermined total inductance, owingto the reduced spacing of the opposed fields of the coils. The design constants previously given for the amplifier of Fig. 2 represents a considerable margin of safetyon the side of stability, this design resulting in a stable amplifier regardless of whether the volume controlis by variation of anode potentialor by variation of grid bias, and regardless of .whether the volume control is applied to the last three tubes or lto all five of the tubes. y 1

Attention is now directed' to Fig. l8' in which the amplification in an amplifier including transformer coupling is plotted as a function of the frequency. The amplification is measured as the ratio of the potential between the control electrodeand cathode of a succeeding tube to that, across the control electrode and cathodeof av preceding tube, the transformer in question serving to couple the two tubes. A biiilar transformer results in a smooth curve like the curve 1E in `Fig. 8. If a transformer is constructed which is not kwound bililarly but in which very closeor substantial unity coupling is attempted a! curve is obtained like the curve F in Fig. 8. An i usuallyhave several inflection points, like the curve G iny Fig. 8. In each case, if the transformers are so as to obtain a ras are usedkin television owing to the high VVative to the modulation Ordin ary transformer will hand, it may be desired to have a made astatic by vbeing dituned to` a frequency of say 6000 videdinto sections, and if the natural resonance frequency of these sections comes within the working frequency range inflection points'will result, such as the points By applying my invention these inflection points -I may be smoothed'out, as 'is' indicated by thedotted lines. l Thesev curves illustrate the extreme advantageof applying the presentv invention to a transformer of the bifilar type, for in an ordinaryk transformer having an amplification curve like the curve G, vfor example-the variations in amplification are so great that a refinement such as I am hereldealing with may not be significant. Of course, ifonly a limited portion of the range will be used smooth curve it would again be desirable to` prevent resonance peaks, caused by making the transformers astatic, in this portion orrange. Y f

Attention is next directed to the vreceiver circuit which is schematically illustratedv in Fig.V 9 of the drawings. This receiver is inist tended for use atvery high frequencies, such ,p

modulation frequency which must be accommodated. Inthis circuit an antenna is coupled toa radio frequency amplifier 102, the output of `which is mixed with constant frequency energyfrom an oscillator 104 in a detector or other device ofasymmetric characteristic106,*so as to reduce the-carrier frequency-"toen intermediate frequency which may be more readily amplified. lated intermediate frequency energy isamplified in an intermediate frequency amplifier 108, and the ampliiiedenergy may be detected in a suitable detector in order to The modu- A,

obtain the modulation rorv light variation component, whichin turn is led'to a suitable l" television reproducer 112, which may include the usual rotating disc for integrating'the light variations into a complete picture.

rlhe original `carrier energy may have a frequency of say 5000 kilocycles, andthe modulation frequency mayy be say 100 kilo-V cycles.v If the intermediate amplifier 108 is automatically tuned, vlike the amplifier de-V ,i

scribed'in'Fig. 1 or more generally in accordance with the teachings of my copending application Serial No. 397,633, 5, 1929, now Patent No. 1,779,881, anda copending application of Jacob Yolles and me,

filed October c Serial No. 403,161, filed October 29, 1929, the v intermediate frequencies may be kept low relfrequency. For eX- ample, the oscillator 104; may have a frequency 'of say 5200 kilocycles, sothat the intermediate carrier frequency vwill be 200 kilocycles, modulated to frequencies ranging between 100 and 300'kilocycles; On Vthe other tuned amplifier for the intermediate amplisimple unfier 108. In such cases the oscillator may be kilocy'cles,

resulting in an intermediate carrier frequency of 1000 kilocycles, modulatedbetween y of screen grid tubes 120, which are coupled by preferably bifilar wound transformers 122. The transformers are preferably made naturally resonant to a frequency lower than the working range of frequency, and are preferably wound astatically in order to deoou ple them from one another to prevent magnetic coupling between the successive stages. In such case the natural frequency of the sections may fall within the working frequency range and cause undesired peaks in the impedance curves of the transformers and in the amplification curve of the amplifier. A gradual slope in the amplifier curve is permissible, and will not cause a distortion in the picture which is readily discernible to the eye, whereas sudden inflections at some particular frequency may cause distortion which will be very apparent to the eye.

To obviate this difficulty the transformers 122 may then be wound with an increased number of sections, there being four in the present case, alternatesections being wound in opposite directions so as to retain and even improve the astaticism of the transformers and at the same time preventing resonance points within the working frequency range.

Each of the input circuits of the tubes may include a series inductance 124 of suitable magnitude to series resonant with the apparent input capacitance of the tubes 120 in order to level out the gain by increasing the gain at high frequencies, in accordance with the disclosure inthe copending application Serial No. 402,378, aforesaid.

It will be apparent that while I have shown and described my invention in the preferred forms, many changes and modifications may be made in the structures disclosed without departing from the spirit of the invention, defined in the following claims.

I claim:

1. The method of making a coil and of preventing undesired resonance peaks in said coil within a predetermined frequency range fixed by external conditions, such as the broadcast frequency band, which includes subdividing the coil into a plurality of series connected sections such that each section has a natural frequency outside of the frequency range.

2. The method of preventing undesired resonance peaks in a coil intended for use over a working frequency range fixed by external conditions, such as the broadcast frequency band, and having a natural frequency lower than the lowest frequency of the range which includes subdividing the coil into a plurality of series connected sections such that each section has a natural frequency higher than the highest frequency of the range 3. The method of preventing undesired resonance peaks in a transformer intended for use over a working frequency range fixed by external conditions, such as the broadcast frequency band, and having a natural frequency lower than the lowest frequency of the range which includes winding the transformer bi filarly, and subdividing the transformer into a plurality of series connected sections ysuch that each section has a natural frequency higher than the highest .frequency ofthe range.

4. The lmethod of preventing undesired coupling with and undesired resonance peaks in a coil intended for use over a working frequency range fixed by external conditions, such asthe broadcast frequency band, and having a natural frequency lower than the lowest frequency of the range which includes subdividing the coil into a plurality of series connected sections such that each section has a natural` frequency higher than the highest frequency of the range, and winding alternate sections in opposite directions.

5. The method of preventing undesired coupling with and undesired resonance peaks in a transformer intended for vuse over a workingv frequency range fixed by external conditions, such as the broadcast frequency band, and having anatural frequency lower than the lowest frequency of the lrange which includes winding the vtransformer bifilarly, subdividing the transformer into a plurality of series connected sections such that each section has a natural `frequency higher than the highest frequency of the range, and winding alternate sectionsl in opposite directions. A coil for use with a predetermined band 6. of frequencies xed by external conditions, said such as the broadcast frequency band,

coil comprising a plurality of sections each naturali-.y resonant to a frequency outside of the band.

7 A coil for use with a predetermined band of frequencies fixed by external conditions, such as the broadcast frequency band, said coil consisting of a plurality of sections arrangedV in series, the coil being naturally resonant to a frequency lower than the lowest frequency of the band, and the vsections being resonant to a frequency higher than the highest frequency of the band.

8. A transformer for use with a predetermined band of frequencies fixed by external conditions, such as the broadcast frequency determined band of frequencies fixed by ex-y ternal conditions, such as the broadcast fre-k plurality of sections arranged in series, the

transformer being naturally resonant to a frequency lower tha-nthe lowest frequency of the band,4and the sections being naturally resonant to a frequency higler than the highest 'frequency of the band. v

10. A iixedtransformer for use with a predetermined band of yfrequencies,such as the radio broadcast'band, said ytransformer comprising bifilar Wound primary and secondary coils divided into a pluralityof sections arranged inseries,the transformer being natvurally resonant toa frequency lower than the lowest frequency of the band, and the sections being naturallyy resonant to a frequency higher than the highest frequency of the band@k v- 'q 11. An astatic coil for use with a predetermined band of frequencies such as the radio broadcast band,said coil comprising a plurality of sections connected in series, alter- .nate sections being Wound in opposite directions in order to make the coil astatic, and each of the sections being naturally resonant to a frequency outside of the band.

12. An astatic coil for use with a predetermined band of frequencies, such as the radio broadcast'band, said coil being naturally res-l onant to a frequency lower than the band,

and comprising a plurality of sections con-l nected 1n series, 4alternate sections being Woundfinopposite directions in order to make the coil astatic, and the number of sections being so selected that each of the sections is naturally resonant to a frequency higher thanthe highest frequency in the band in order to prevent resonance peaks in the band. Y

` 13. An astatic'transformer fo-r use With a predetermined band of frequencies such 65.predetermined rband of frequencies, such as as the radio broadcast band, said coil comprising` very closely coupled primaryl and secondary' coils divided into a plurality of sections connected in series, alternate sections being Wound in opposite directions in order to make the transformer astatic, and the number of .sections being so selected that each of the sections is naturally resonant to a frequency higher than the' highest frequency in the band. Y

14. An astatic transformer for use With a predetermined band of frequencies, such as the radio broadcast band, said transformer being naturally resonant to a. frequency lower than the lowest frequency in theband, and comprising a plurality of sections connected in series, alternate sections being Wound in opposite directions Vin" order to kmake the transformer astatic, the number of secticns being so selectedthat each of the sections is naturally vresonant to a frequency higher than the highest frequency in the band.

15. An astatic transformer for iisewvith a ondary coils divided into a plurality of seriesV connected sections, alternate sections being wound in opposite directions in order to'make the transformer astatic, and the number of sections being so selected that .each of 'the sections is naturally resonant to a frequency higher than the highest frequency in the band in order t-o prevent resonance peaks in the band.,A Y f Y 16. An amplifier for use over a predetermined range of frequencies such as the broadcast band,rsaid amplifier comprising a plurality of. electron emission tubes, coils for coupling said tubes in cascade, said coils consisting of sections so 'selected that thek natural frequency of each of the secti-onsfis outside ofthe frequency range.

1'?. An amplifier for use over a predetermined range of frequencies such as the broadcast band, saidfamplilier comprising a plurality of electron emission tubes, coils for coupling said tubes in cascade, said coils comprising a 'plurality of sections connected in series and Wound in opposite directions in order to make the coils astatic, the number of sections being so selected thaty then'atural frequency of each ofthe sections ishigher than the highest'frequency of the frequency range.

18. An amplifier for use over a predetermined range of frequency such as the broad* cast band, said amplifier comprising a plu-V cast band, said amplifier comprising a plu-` rality of electron emission tubes, a transformer for coupling. said tubes in cascade, said transformer comprising bifilarvvound primary and secondary coils divided into a eluralityV of sections connected inseries and Wound in opposite directions in order to make the transformer astatic, the natural frequency of each of the sections being higher than the highest frequency of the Working frequency range.

20. An automatically tuned amplifier for use overa predetermined range of frequencies such as the broadcast band, said amplifier comprising a pluralityof electron emission tubes, transformers for coupling` said tubes in cascade, said transformers being successively naturally resonant in circuit to dierent decreasing requencies, said transformers oomprising biilar Wound primary and secondary v coils divided into a plurality of sections connected in series and Wound in opposite directions in order to make them astatio, the natural frequency of each of the sections being higher than the highest frequency of the frequency range. Signed at New York in the county of New York and State of New York this 21st day of October, A. D. 1929.

LESTER L. JONES. 

